1. Field of the Invention
The present invention relates to a method for producing a structured substrate, a structured substrate, a method for producing a semiconductor light emitting device, a semiconductor light emitting device, a method for producing a semiconductor device, a semiconductor device, a method for producing a device, and a device, in particular, to those suitable for producing a semiconductor laser, a light emitting diode, or an electron traveling device using for example a nitride type III-V group compound semiconductor.
2. Description of the Related Art
Nitride type III-V group compound semiconductors such as GaN, AlGaN, GaInN, and AlGaInN feature in a large band gap Eg and direct transition semiconductor materials in comparison with arsenic type III-V group compound semiconductors such as AlGaInAs and phosphorous type III-V group compound semiconductors such as AlGaInP. Thus, these nitride type III-V group compound semiconductors have attracted considerable attention as materials of semiconductor lasers that can emit short wavelength light ranging from ultraviolet ray to green and materials of semiconductor light emitting devices such as light emitting diodes (LEDs) that can cover a wide range of light emitting wavelengths from ultraviolet ray to red and white. These materials are expected for wide applications such as high density optical discs, full color displays, environmental and medical fields.
In addition, these nitride type III-V group compound semiconductors for example GaN feature in a large saturation speed in a high electric field, a high temperature operation of for example up to around 400° C., and continuous crystal growth for a semiconductor layer and an insulation layer using AlN in for example a metal-insulator-semiconductor (MIS) structure. Thus, these nitride type III-V group compound semiconductors are expected for materials that compose radio frequency electronic devices that can operate at high temperature and with a large output.
In addition, these nitride type III-V group compound semiconductors have the following advantages.
(1) Since they have higher thermal conductivities than GaAs type semiconductors, they are suitable for devices that operate at high temperatures and with large outputs.
(2) Since they are chemically stable and hard, they have good reliability.
(3) They are compound semiconductor materials that less contaminate environment. In other words, AlGaInN type semiconductors do not contain environmental pollutants and poisonous substances. In reality, they do not contain arsenic (As) for AlGaAs type semiconductors, cadmium (Cd) for ZnCdSSe type semiconductors, and a material arsine (AsH3).
However, proper substrate materials for devices using nitride type III-V group compound semiconductors that have good reliability are not known.
To obtain high quality crystals, substrate materials for nitride type III-V group compound semiconductors have the following problems and conditions to be solved and satisfied.
(1) Structural materials GaN, AlGaN, and GaInN of the nitride type III-V group compound semiconductors are of full distortion type of which there are different lattice constants. Thus, compositions, thicknesses, and so forth of nitride type III-V group compound semiconductors and substrates should be designed so that they are free from cracks and obtain good crystal films.
(2) A high quality substrate that can lattice-match GaN has not been developed. Like a high quality GaAs substrate that can lattice-match a GaAs type semiconductor and a GaInP type semiconductor and a high quality InP substrate that can lattice-match a GaInAs type semiconductor, for example a high quality GaN substrate is under development. A SiC substrate having a relatively small difference of lattice constants is expensive. In addition, it is difficult to produce a SiC substrate having a large diameter. Since a tensile distortion takes place in a crystal film, it easily cracks. In addition, there is no substrate that can lattice-match GaN other than those.
(3) Necessary conditions of substrate materials for nitride type III-V group compound semiconductors are a high crystal growth temperature of around 1000° C. and no deterioration and no corrosion of V group materials in an ammonium atmosphere.
In consideration of the foregoing reasons, as a substrate of a nitride type III-V group compound semiconductor, a sapphire substrate is often used.
A sapphire substrate is stable at crystal growth temperature of a nitride type III-V group compound semiconductor. Thus, as an advantage, high quality substrates having a diameter of two inches or three inches can be stably supplied. However, lattice-mismatch of a sapphire substrate to GaN is large (around 13%). Thus, a buffer layer made of GaN or AlN is grown on the sapphire substrate at low temperature. Above the buffer layer, a nitride type III-V group compound semiconductor is grown. As a result, although a single crystal of a nitride type III-V group compound semiconductor can be grown, the defect density is as large as 108 to 109 (cm−2) due to lattice mismatching. Thus, when the nitride type III-V group compound semiconductor is used for a semiconductor laser, it does not have reliability for a long time.
In addition, (1) since a sapphire substrate does not have cleavage, an end plane of a laser cannot be stably formed with specular property. (2) Since sapphire is insulative, it is necessary to take out a p-side electrode and an n-side electrode from the upper surface of the substrate. (3) When a crystal growth film is thick, due to the difference of thermal expansion coefficients of a nitride type III-V group compound semiconductor and sapphire, the substrate largely skews at room temperature. As a result, the device forming process is adversely affected.
To obtain a high quality semiconductor crystal that is grown on a substrate such as a sapphire substrate whose lattice constant is different from the semiconductor crystal, a method using epitaxial lateral overgrowth (ELO) is known. In the ELO, high crystal quality regions (lateral growth regions) and low crystal quality regions (or high defect density regions) (on seed crystals, their boundaries, meeting portions, and so forth) periodically take place. However, when the size of an active region (for example, a light emitting region of a light emitting device or an electron traveling region of an electron traveling device) is not large, the period of the ELO can be greater than the interval of stripes of a semiconductor laser and the interval of emitter region/collector region (or source region/drain region) of a transistor. For example, the period of the ELO is 10 to 20 μm, whereas the size of an active region of a device is around several μm. Thus, an active region can be designed to be formed in a high quality region.
When a device is formed on a sapphire substrate by the ELO, in addition to the foregoing problem of bad cleavage due to characteristics of sapphire, there are for example the following problems.
(1) Since the number of steps necessary for the ELO is large, the yield decreases.
(2) Since the crystal film thickness increases for the ELO, the substrate largely skews due to thermal stress. As a result, the controllabilities of the crystal growing step and wafer process deteriorate.
(3) The device size is restricted. A device such as an LED, a photo detector (PD), and an integrated circuit device that have an active region greater than the ELO period (namely, one side of the active region is for example several hundred μm), since all the device region cannot be formed as high crystal quality regions, the effect of the ELO cannot be fully obtained.
Although the foregoing problems would be solved when a high quality GaN substrate could be obtained. However, so far, a high quality GaN substrate having a large diameter has not been obtained. This is because a good seed crystal cannot be obtained from GaN by hydride vapor phase epitaxy (HVPE), which is high temperature (high pressure) growth. Thus, single crystal growth cannot be stably performed. As a result, a high quality substrate cannot be easily produced.
Japanese Patent Laid-Open Publication No. 2001-102307 has proposed a method for producing a single crystal GaN substrate so as to solve the foregoing problems. According to the related art, after a GaN seed substrate having a high defect density is formed, a three-dimensional facet as a core is formed at a part thereof. A crystal is grown so that the facet is not closed. Crystal dislocations are gathered around the core portion. As a result, a wide substrate having high quality is produced.
However, that technology disclosed in Japanese Patent Laid-Open Publication No. 2001-102307 causes the through-dislocations to be gathered around a region of a growth layer so as to decease the through-dislocations of the other regions. Thus, a low defect density region and high defect density regions coexist in the obtained single crystal GaN substrate. In addition, the positions of the high defect density regions cannot be controlled. Instead, the high defect density regions randomly take place. Thus, when a semiconductor device for example a semiconductor laser is produced, a nitride type III-V group compound semiconductor layer is grown on a single crystal GaN substrate. At that point, a high defect density region cannot be prevented from being formed in a light emitting region. As a result, light emitting characteristics and reliability of the semiconductor laser deteriorate.